It's a jungle out there. And in here. We all know that animals play a role in medicine. But you may not have known what a wide range of animals are finding their way inside the human body--sometimes literally, in the form of transplanted tissue, but more often figuratively, in the way they help researchers develop treatments for humans. Our furry (and scaly, and feathered) friends are saving more lives, with each passing year. From apes to zebrafish, here's a sampling of the new medical menagerie.

One day, we may be part-salamander. Or something like that, at least--scientists are hoping the unlock the salamanders limb-regenerating ability in humans. In a recent study in the oddly-named journal, Cell Stem Cell, Helen Blau et al. reported that they found a way to recreate that ability in mouse muscle cells. The scientists focused on a tumor-suppressing gene called Arf, present in mammals but not in regenerating vertebrates. By silencing that gene, they were able to get mouse cells to reenter the cell cycle. But yes, you read that right--the gene is involved in tumor suppression. "There's no question we're playing with fire," said one researcher, Jason Pomerantz, to Nature News. More study is, as they say, needed.

Pigs are already saving and improving human lives (with some help from doctors). Pig tissue helps people recovering from all sorts of ailments and procedures, from hernias to plastic surgery. Pig skin and pig valves are becoming something of a commonplace in surgical medicine, but there's one barrier we haven't yet crossed: the transplantation of an entire animal organ. Scientists in Melbourne, Australia recently used a ventilator and pump to keep pig lungs alive and essentially "breathing" as human blood coursed through them, reportsThe Telegraph, leading experts to predict an out-and-out organ transplant within five years. There's a fun word for these cross-species operations: "xenotransplantation." Medical ethicists have hit pay dirt: "It is about whether the community is prepared to accept a part human, part animal," one told The Telegraph.

Milk does a body good. So does cow tissue, which can help heal wounds. Some researchers advocate for our spotted friends to have a role in wound treatment. According to one source, a paper presented last year by the National Institute for Materials Science in Japan found that bovine collagen "provides the perfect material for clinical scaffolds to repair damaged articular cartilage."

A team of Vanderbilt researchers have found that zebrafish might be a fruitful platform for future drug development. Zebrafish embryos are a nice sort of medical research sandbox in which to help identify compounds that could point researchers towards ideas for treatments for bone diseases and cancer.

The little woolly beasts, it turns out, are excellent model organisms for studying the heart. Researchers use them to test cardiac devices and artificial heart valves. “Sheep hearts are similar in size to human hearts,” a researcher toldMinnesota Medicine back in 2007. “Their valves calcify at an accelerated rate, so in six months we can model what takes decades to happen in humans.”

The modest mouse. No medical research lab would be complete without the humble rodent. Up to 90% of animals used in biomedical research are rodents, in fact, and mice make good models because of how similar their DNA is to ours. "Mice are unpretentious, it is very easy to feed them, they breed fast and they are small in size," reads part of an encomium published last year on the occasion of the 100th anniversary (give or take) of the lab mice industry.

Animal research, of course, is controversial. Great apes play a role in medical research in the United States--over 1,000 chimps are used for research in the U.S.--but they may not for long. The Great Ape Protection Act gained new ground in Congress last summer, but is still pending. Other countries, including several in Europe, have banned medical testing on chimps, bonobos, gorillas, and orangutans on ethical grounds, saying that they are too cognitively similar to our own species. As the line between humans and animals blur--either because we are appropriating their organs in a hybrid-like way, or because we understand that some creatures aren't so different from us--the new medical menagerie becomes an increasingly fraught one.

The New Medical Menagerie

It's a jungle out there. And in here. We all know that animals play a role in medicine. But you may not have known what a wide range of animals are finding their way inside the human body—sometimes literally, in the form of transplanted tissue, but more often figuratively, in the way they help researchers develop treatments for humans. Our furry (and scaly, and feathered) friends are saving more lives, with each passing year. From apes to zebrafish, here's a slideshow of the new medical menagerie.